For persons with impaired hearing, the hearing aid devices which are most commonly used today are those based on the principle that the sound is amplified and fed into the auditory meatus and stimulates the eardrum from the outside. In order to prevent acoustic feedback problems in these devices, the auditory meatus is almost completely plugged by a hearing plug or by the hearing aid device itself. This causes the user a feeling of pressure, discomfort, and sometimes even eczema. In some cases it even causes the user problems like running ears due to chronic ear inflammations or infections in the auditory canal.
However, there are other types of sound transmitting hearing aids on the market, i e bone anchored hearing aids which mechanically transmit the sound information to a persons inner ear via the skull bone by means of a vibrator. The hearing aid device is connected to an implanted titanium screw installed in the bone behind the external ear and the sound is transmitted via the skull bone to the cochlea (inner ear), i e the hearing aid works irrespective of a disease in the middle ear or not. The bone anchoring principle means that the skin is penetrated which makes the vibratory transmission very efficient.
This type of hearing aid device has been a revolution for the rehabilitation of patients with certain types of impaired hearing. It is very convenient for the patient and almost invisible with normal hair styles. It can easily be connected to the implanted titanium fixture by means of a bayonet coupling or a snap in coupling. One example of this type of hearing aid device is described in U.S. Pat. No. 4,498,461 and it is also referred to the BAHA® bone anchored hearing aid marketed by Entific Medical Systems in Göteborg.
Other types of bone conducting hearing aids are described in U.S. Pat. No. 4,904,233 and in Swedish patent application 0002071-9.
A common feature for the hearing aid devices which have been described so far is that some type of vibratory generating means, vibrators, are required. Different types of vibrators are well known in the art. There are a number of known vibrator principles today. In traditional as well as in bone anchored hearing aid devices it is normally used a vibrator principle which was described already by Bell in 1876. There is a detailed description of this principle applied on a bone anchored, bone conducting hearing aid device in “On Direct Bone Conduction Hearing Devices”, Technical Report No. 195, Department of Applied Electronics, Chalmers University of Technology, 1990. Other vibrators of this type are described in Swedish patents 0002072-7 and 0002073-5.
In order to improve the sound quality and reduce the risk for acoustic feed back problems in the hearing aid it is necessary to damp the resonance frequency of the vibrator, i e the resonance frequency which is generated by the mass-spring system, which consists of the counter-acting mass (including coil, magnet etc), and the inner spring in the vibrator. In conventional bone conductors there is no need for any internal damping of this frequency as the skin between the vibrator and the bone has a damping function in itself. When the vibrator is connected directly to the bone, however, a significant resonance peak is generated in the frequency response characteristics which gives a poor sound quality and feed-back problems.
The above-mentioned problems with the bone anchored hearing aids can be solved by providing the vibrator spring with some kind of mechanical damping. Then the original design of the spring has to be changed significantly, for instance it is changed into a sandwich structure in the form of a damping material applied between thin plates.
In this context it is referred to Swedish Patent No. 85.02426-3 in which it is illustrated a vibrator comprising a vibrator plate and a coil which is wound around a bobbin base having a core and two side walls. It also comprises means for damping the resonance frequency of the vibrator in the form of a spring provided with a layer of a damping material or a built-in damping material.
Also other types of mechanical damping means have been proposed, for instance ferro-fluid damping as described in Swedish patent application 0102206-0. In this case the gap between the vibrator plate and the bobin base, or some other spacing in the vibrator in which a relative movement between two surfaces is generated during the vibratory function, is at least partially filled with a fluid or a gel. The purpose of this fluid or gel is to provide the main part of the damping of the resonance frequency of the vibrator. Preferably the fluid or gel comprises ferro-magnetic particles, a so-called ferro-fluid, in order to keep the fluid in place and increase the magnetic conductivity in the magnetic circuit.
It has turned out that these types of vibrators with mechanical damping means in the form of a damping spring or a damping fluid not always give an optimal function of the hearing aid. The damping spring is a mechanically complicated and exposed part in the hearing aid and the ferro fluid damping is also a rather complicated technical solution.
A vibrator spring with an integrated damping in the form of a damping material or mass has also a number of disadvantages. In the first place, the damping material not only has a damping function but it also gives the spring a more uncontrolled stiffness. This is a serious disadvantage as the spring stiffness is a sensitive parameter in this type of vibrator. If the spring is too weak there is a risk for collaps, on the other hand if the spring is too rigid it has a negative effect on the overall performance. Furthermore, the damping material has a stiffness which depends on the temperature which means that the performance is seriously effected by temperature changes. If the temperature is too low, the vibrator is significantly weaker.
Also, the damping mass has a frequency dependent stiffness which means that the spring becomes more stiff at audio frequencies. This is quite in contrast to what you want as it gives an unnecessarily high resonance frequency compared to the case with no damping mass at all, which means that approximately twice as high weight has to be used in order to obtain the same resonance frequency. This is of course not acceptable in the case of small, compact devices.
Secondly, when the damping material is growing older it has a negative effect on the vibratory performance due to the fact that the resonance frequency increases with the increased stiffness of the damping mass. From a manufacturing point of view the introduction of a damping mass is not what you want. Even the reliability of the vibrator is seriously effected by the damping mass as there is a tendency that the damping mass will be creeping away after the manufacturing process which means a risk for collapse of the vibrator.
A further disadvantage with the mechanical damping is the fact that the degree of efficiency is decreased. In order to dampen the resonance peak a valuable amount of battery power is consumed just in the form of heat generation in the damping mass.